A significant chunk of the West Antarctic Ice Sheet has begun to disintegrate and, owing to the ice sheet’s peculiar topography (much of it lies below sea level), this process, having begun, has now also become unstoppable. “Today we present observational evidence that a large section of the West Antarctic Ice Sheet has gone into irreversible retreat,” the lead author of one of the papers, Eric Rignot, a glaciologist atNASA’s Jet Propulsion Laboratory, said at a news conference. “It has passed the point of no return.” Rignot said that melting in the section of West Antarctica that his team had studied could cause global sea levels to rise by four feet over the course of a couple of centuries. . .

In more recent years, even as forecasts of global sea-level rise have been notched up, most projections have not taken into account the possibility of a significant, near-term ice loss from the West Antarctic. The most recent analysis by the U.N.’s Intergovernmental Panel on Climate Change forecasts a global sea-level rise for this century of somewhere between one and three feet; the new findings, according to Rignot, will require these figures to be revised upward. . . .

In an interesting paper that appeared in the journal Global Environmental Change, a group of scholars, including Naomi Oreskes, a historian of science at Harvard, and Michael Oppenheimer, a geoscientist at Princeton, note that so-called climate skeptics frequently accuse climate scientists of “alarmism” and “overreacting to evidence of human impacts on the climate system.” But, when you actually measure the predictions that climate scientists have made against observations of how the climate has already changed, you find the exact opposite: a pattern “of under- rather than over-prediction” emerges. The scholars attribute this bias to the norms of scientific discourse: “The scientific values of rationality, dispassion, and self-restraint tend to lead scientists to demand greater levels of evidence in support of surprising, dramatic, or alarming conclusions.” They call this tendency “erring on the side of least drama,” or E.S.L.D. for short.